1. Field of the Invention
The present invention relates to an electrode structure for polymer electrolyte fuel cells, and polymer electrolyte fuel cell using the same.
2. Description of the Related Art
The environmental problems, e.g., global warming, resulting from consumption of fossil fuels are becoming more serious, while oil resources are being depleted. Therefore, fuel cells have been attracting attention as clean power sources for motors, which release no carbon dioxide. They have been extensively developed, and commercialized in some areas. When a fuel cell is mounted in an automobile or the like, a polymer electrolyte fuel cell including a polymer electrolyte membrane is suitably used, because of its capacity to produce a high voltage and large electric current.
As an electrode structures for polymer electrolyte fuel cell, one comprising a pair of electrode catalyst layers composed of a catalyst of platinum or the like supported by a catalyst carrier of carbon black or the like and monolithically formed by an ion-conductive polymer binder, and an ion-conductive polymer electrolyte membrane placed between these electrode catalyst layers in which each electrode catalyst layer is coated with a diffusion layer (see, e.g., by Japanese Patent Laid-Open No. 2000-223136). The electrode structure constitutes a polymer electrolyte fuel cell with a separator, which also works as a gas passage, laminated on each of the electrode catalyst layers.
In the polymer electrolyte fuel cell, a reducing gas, e.g., hydrogen or methanol, is introduced into one electrode catalyst layer working as the fuel electrode via the diffusion layer, and an oxidizing gas, e.g., air or oxygen, is introduced into the other electrode catalyst layer working as the oxygen electrode also via the diffusion layer. This structure produces proton from the reducing gas on the fuel electrode by the actions of the catalyst in the electrode catalyst layer. The proton moves towards the electrode catalyst layer on the oxygen electrode side via the polymer electrolyte membrane, to react with the oxidizing gas introduced on the oxygen electrode to produce water in the electrode catalyst layer on the oxygen electrode side by the actions of the catalyst contained in the catalyst layer. Therefore, a current can be produced by connecting the fuel electrode and oxygen electrode to each other by a cable.
For the polymer electrolyte membrane in the electrode structure, perfluoroalkylenesulfonic acid polymer compounds (e.g., Du Pont's Nafion (trade name)) have been widely used. The perfluoroalkylenesulfonic acid polymer compound exhibits good proton conductivity and chemical resistance as a fluorine-based resin, because it is sulfonated.
However, perfluoroalkylenesulfonic acid polymer compound involves some disadvantages. It is very expensive, and low in creep resistance at high temperature and hence difficult to stably retain its shape. Another disadvantage results from its high gas permeability, which may deteriorate its durability, in particular under high temperature/low humidity conditions, because oxygen may cross-leaks from the cathode side to react in the vicinity of the catalyst on the anode side to produce hydrogen peroxide as a by-product, which will accelerate deterioration of the membrane.